* CBO raises estimate by 50 percent
* Cites economic slowdown, state opposition to Medicaid
WASHINGTON, Sept 19 (Reuters) - U.S. budget experts raised their forecast on Wednesday of how many Americans will probably have to pay a penalty in 2016 for not buying health insurance to 6 million from 4 million.
The 50 percent increase was likely to draw fire from Republicans on the campaign trail who want to repeal President Barack Obama's signature healthcare law and who reject the penalty as a government intrusion into the lives of individuals.
But the nonpartisan Congressional Budget Office said some of the increase reflects state opposition to an expansion under the healthcare law of the Medicaid program for the poor, which is most unpopular in states with Republican governors or Republican-majority legislatures.
CBO, which issued its last forecast in April 2010, also attributed the larger number of people facing penalties to a bleaker economic picture that will mean higher unemployment and lower wages and salaries.
There are now 49 million people without health insurance in the United States, according to the U.S. Census Bureau.
Under the Affordable Care Act, better known to the public as "Obamacare," more than 30 million people would become eligible to buy subsidized private insurance or receive Medicaid coverage in 2014.
The law requires most Americans to have some form of health insurance - known as the individual mandate. The law stipulates that those who do not acquire health coverage will face a penalty.
The penalty is scheduled to rise in 2016 to $695 or 2.5 percent of household income, whichever is greater. That year is when the law's provisions are expected to operate fully.
The government is expected to collect between $7 billion and $8 billion in revenue from the penalty, which the Supreme Court ruled constitutional as a form of taxation earlier this year.
The CBO projects that 30 million people who are not elderly will still be uninsured in the United States in 2016. But most will not be subject to the penalty because they are illegal immigrants, members of exempted groups including Indian tribes or have very low incomes.